i like to open my rein on the jump on the side i want the lead.
sometimes looking in the direction you are about to turn helps sometimes... also using an inside leg to push them out and to pull a little with inside reign!
Whoops forgot to say, also have your leg on as if you're asking for a flying change in the air.
Keep an opening rein, look in the direction you're going!
Say your landing left you turn his nose left with outside leg and if your landing right pull his nose slightly more to the right and outisde leg. My old pony barely did leadchanges
You need to ask the lead while jumping. When above the jump, look in the direction you want to go, indicate the direction with you hand (following your horse's mouth obviously), you can move you external leg back. If you horse end up in the wrong lead, do a quick transition to trot and ask canter again.
Good luck :)
Look in the direction you want to go and try opening your inside hand a little!